Fungi I - Evolution and Diversity, Phyla Chytridiomycota and Zygomycota

Introduction and Goals

This tutorial will cover the vast and variable Kingdom Fungi, and its impact on the environment. Fungi (fungus, sing.), like bacteria, are most commonly decomposers, assisting in the essential task of recycling nutrients in ecosystems (Fig. 1). The majority of all plant species depend on symbiotic fungi for enhanced water and nutrient absorption. Other species have been exploited by humans for centuries for the production of bread, beer, wine, cheese, and edible fruiting bodies (mushrooms). Antibiotic-producing fungi (e.g., Pennicillium chysogenum) have changed the face of modern medicine, saving millions of lives. Conversely, other species cause economically important and often devastating diseases to plants and animals (humans included). Fungal-induced plant disease epidemics have had dramatic historical influences, resulting in starvation, war, and human migration. Fungi are also responsible for the rots that damage timber, agricultural products, and human-made structures. Some fungi produce conspicuous fruiting bodies in a variety of shapes and sizes (Fig. 2).

Figure 1. A club fungus (Click image to enlarge)

Figure 2. A cup fungus (Click image to enlarge)

By the end of this tutorial you should have a fundamental understanding of:

The roles and importance of fungi in the biosphere

Basic fungal nutrition

Basic morphological diversity within the kingdom

Basic reproductive features of fungi

Performance Objectives(these are the same for Tutorials 12 and 13):

Identify the general characteristics of the fungi

Explain how the “body” of the fungus is adapted for its mode of nutrition

Diagram the general fungal life cycle and describe the difference between plasmogamy and karyogamy

Summarize the basic classification of fungi

Describe the different types of symbiotic relationships that fungi participate in

For each taxonomic group of fungi, identify the major characteristic(s) of that group and be able to discuss representatives that demonstrate the diversity of the group

Explain different ways in which fungi impact humans, either directly or indirectly

Basic Biology and Morphology

Although there are now more than 100,000 described species of fungi, mycologists (scientists who study fungi) estimate that there are probably more than 1.5 million species. The kingdom is comprised of six major groups:

Microsporidians

Chytridiomycota

Zygomycota

Glomeromycota

Ascomycota

Basidiomycota

The categorization of fungi is based on molecular data from DNA sequences and life cycle features. In some cases, a complete sexual life cycle has not been observed and some mycologists use a fifth phylum (Imperfecta or Deuteromycota) as a repository for these species, but with the information now available from molecular data this category is rarely used..

Although originally grouped with plants, modern molecular systematists now think the lineage that gave rise to modern fungi arose before plants; the molecular data suggest that this occurred just over one billion years ago. The amoeboid protists are the sister group to fungi and animals.

All fungi are heterotrophic, and many are important and prolific decomposers, recycling organic carbon trapped mostly in plant cell walls as lignin and cellulose. A major distinguishing feature of the kingdom is absorptive nutrition. Food sources are digested externally, then absorbed into their cells. They produce large amounts of organic acids to acidify their local environment, followed by powerful digestive enzymes to break down their food source. Fungi are almost unlimited in what they can digest. Everything from trees to human skin (e.g., athlete's foot, ringworm) are subject to fungal decay.Fungi are also distinguished by their filamentous vegetative structures, hyphae (hypha, sing.; somewhat different structurally from the hyphae of the oomycetes discussed in Tutorial 9), and the presence of chitin as a primary cell wall polymer. A group or mass of hyphae (mycelium) may be visible to the naked eye as a white or gray fuzz on moldy bread or on a decaying tree.

Extensive networks of mycelium give fungi a tremendous amount of surface area in a small space, making them ideally adapted to their absorptive mode of nutrition. Cellular specialization is somewhat limited in the kingdom. As depicted in Figure 3, hyphae can be characterized as having cells clearly delimited by structures called septa (septate hyphae), or the absence of septa (coenocytic hyphae). How is this body structure an adaptation to absorptive feeding?

Many fungal plant parasites invade host tissue with specialized structures (haustoria) that are used to absorb the cellular contents of their hosts (Figure 4). These fungi provide no benefit to the plant.

Environmental Impact

Members of the kingdom Fungi produce spores. Although they are mostly nonmotile (some members of the Chytridiomycota produce flagellated spores), spores (produced by the thousands or millions) can be dispersed by water and/or wind great distances to another available substrate or host. Spores are an important taxonomic character and, as expected, come in an array of shapes and sizes. Spores can be mitotically or meiotically produced, affording the organism flexibility to reproduce and proliferate over a variety of environmental conditions where sexual recombination may not be feasible.

Fungi play many roles in the biosphere. As you have already learned, they are important decomposers as well as plant and human pathogens. They also engage in extremely important symbiotic relationships with plants and algae. Mycorrhizae (Fig. 5) are an intimate association (symbiosis) between plant roots and fungal hyphae. The vast majority are mutualistic, with the plant or tree benefiting with increased water and nutrient uptake and the fungus obtaining carbon from the roots. All of the members of the glomeromycetes are participants in a mycorrhizal relationship with a plant.Symbioses with lichens are also common. Specific fungal species (usually Ascomycetes) grow in concert with green algae, or occasionally a cyanobacterium. The fungus gains carbon from the photosynthetic partner, and in turn it provides moisture in a controlled parasitic relationship. Technically, it's not a mutualistic relationship since the alga can live on its own and is consumed by the fungus; however, it is a unique relationship since lichens can grow in extremely harsh climates (e.g., the arctic tundra and bare rocks) in which the algae or bacteria might not normally survive. the Cordyceps genus of fungi exhibits a novel method of entomopathogenesis, hijacking the nervous systems of certain insects. Watch this video to see the fungus in action. What effect do they fungi have on they ants they parasitize? How does these behaviors help the fungus?

Basic Biological and Reproductive Features

Fungi exhibit several different types of life cycles. This generalized diagram (Fig. 6) depicts the typical sequence of events in the life cycles of fungi. Focus on the "mycelium (n)" box for a starting point for both the sexual and asexual life cycles. Note, many species can produce asexual spores by mitosis in specialized spore-producing structures. This allows the organism to clone itself while, often times, producing very large numbers of asexual spores. The hyphae of many species are haploid during the majority of their life cycles. Many fungi spend a good portion of their life in the asexual mode. The transition to the sexual mode can be triggered by certain conditions (e.g., light, temperature, moisture, availability of a sexually compatible partner, and limited nutrient availability).

Figure 6. The fungal life cycle (Click image to enlarge)

The sexual stage is characterized by several features that are unique to the kingdom Fungi. Most fungal mating types are morphologically indistinguishable and so, they are not referred to as male or female (fungi also do not produce gametes, sperm or eggs). Rather, they are simply referred to as "+" or "-" to let one know they are different mating types (sexually compatible). In the case of sexual reproduction, compatible mating types fuse in a process that involves plasmogamy (fusion of cell membranes). This fusion produces a heterokaryon (a mycelium with multiple nuclei from the two mating types) or dikaryon (a mycelium with two nuclei from the two mating types; these are found in Ascomycetes and Basidiomycetes), which can divide in the growing mycelium for a prolonged period; we will use the term dikaryon in this discussion of the life cycle. The prolonged dikaryotic stage is a feature unique to fungi. It is thought that the dikaryotic state provides the organism with genetic flexibility in a competitive environment while, at the same time, conferring it with many of the same advantages as a diploid because it possesses a complementary set of chromosomes. The dikaryotic state can last for months, or even years, while the fungus continues to grow and proliferate in its environment.When environmental conditions are suitable, two haploid nuclei fuse (karyogamy) to form a highly transient diploid state. Meiosis follows, almost immediately, in a specialized spore-producing structure, and genetically distinct haploid spores are produced.

Fungal Diversity and the Phylum Chytridiomycota

Modern fungal systematists have divided the fungal kingdom into six major phyla. Figure 7 shows a simplified fungal phylogeny based on molecular data.

We will start with the most primitive members of the kingdom, the Chytridiomycota. Many Chytrids are aquatic, which is why they are often called "water fungi." This phylum has only recently been determined to belong to Kingdom Fungi, therefore, not all textbooks include it in this kingdom. Biologically, the approximately 1000 species are mostly saprobes (introduced in Tutorial6; decomposers that absorb nutrients from dead organic matter), but some species are parasites of animals, plants, or protists (Fig. 8). They are also characterized as having absorptive nutrition and they can have septate or coenocytic mycelium, or be unicellular. Morphologically, they are important because some members possess flagellated spores. Until recently, systematists believed that the absence of flagellated cells was required for placement in the fungal kingdom. Molecular data have shown that chytrids are indeed fungi. Ultrastructural and biochemical data (e.g., chitinous cell walls) support this designation as well. The molecular data also support the theory that they are the most primitive group in the kingdom; they retain the ancestral character of having flagellated cells similar to their protistan ancestors. They are not a monophyletic group (see Fig. 7).

Microsporidians

The microsporidians are another recent addition to the Kingdom Fungi. They are all intracellular parasites of animals or protists, mainly affecting invertebrates (Fig. 9). They do not have mitochondria, but they do have mitosomes. What group do you think they were previously classified in (hint: think back to the Protists)? Microsporidians have no means of locomtion, and are able to form spores that can live outside of the host cell for long periods of time. Human microsporidiosis usually affects only people with compromised immune systems. There is research under way to see if a mosquito-specific microsporidian could be used as a way to control malaria.

The Zygomycetes

Of the roughly 900 described species in the Zygomycota, most are terrestrial and exist as saprobes in soil, on decaying plant material or on animal dung (Fig. 10). Many common bread molds (e.g., Rhizopus stolonifer) are zygomycetes. Morphologically, they have coenocytic hyphae, with septa formed only in association with reproductive structures. The phylum gets it name from the production of the zygosporangium.

Figure 11 illustrates the events in the life cycle of a zygomycete. Concentrate on the top portion of the diagram. As you have learned, two different haploid mating types are often required for sexual reproduction in the fungi. First, gametangia begin to form on hyphae of different mating types, “+” and “-“ (step 1). The gametangia then fuse (step 2) to form the heterokaryotic state (step 3). The heterokaryotic zygosporangium then develops (step 4). The zygosporangium develops a rough and thickened cell wall, which renders it resistant to harsh "over-wintering" conditions. When conditions become favorable for zygospore germination, the nuclei fuse (karyogamy) and a diploid is briefly formed (step 5). Meiosis immediately follows and millions of haploid zygospores are formed in the sporangium by mitosis (step 6). The zygosporangium germinates and releases the spores and the cycle begins again (step 7). The asexual stage alternates between mycelial growth and asexual spore production (as shown in bottom part of figure).

Figure 11. Zygomycete life cycle. (Click image to enlarge)

Phylum Glomeromycota

At one time, the glomeromycetes were considered to be members of the Zygomycota. However, based upon molecular data, the almost 250 species known are now recognized as a monophyletic group of fungi whose members are all mycorhizzae of plants. They form a specific type of structure, an arbuscle, so are called the arbuscular mycorrhizal (AM) fungi. The arbuscle lives within the cell of the plant root, so it is termed an endophytic mycorrhizae (Fig. 12). A mutualistic relationship has been discovered between an AM fungus living in the thallus of a liverwort, a member of the oldest group of land plants. This mutualism is thought to have been very important for the colonization of land by plants and is necessary for the function of terrestrial ecosystems (over 90% of living plants have mycorhizzae).

Summary

This tutorial introduced the fungi. Fungi have an important saprophytic role in the biosphere. You might not find the subject of decomposition pleasant, but without the fungi (and other decomposers) a good proportion of nutrients would remain tied up in dead organisms and never get recycled back into the biosphere.

Fungi obtain their nutrition by secreting various hydrolytic enzymes into their environment. They then absorb the resulting monomeric units. This mode of nutrition is known as absorptive nutrition and is intimately related to the role that these organisms play in degrading organic matter.

The basic aspects of fungal life cycles were also introduced. Not all fungi have a sexual mode of reproduction, but when they do, the haploid state is dominant. An important feature that fungi demonstrate is the presence of a heterokaryotic or dikaryotic state, in which multiple haploid nuclei exist in the same cell. When these nuclei fuse, the resulting diploid nucleus undergoes meiosis very soon afterward to give rise to haploid spores. Thus, the diploid state (as defined by the presence of a true diploid nucleus) is very transient.

The hypha is the prominent cell type in the fungal kingdom. Hyphae can exhibit tremendous growth rates via tip growth, whereby new cellular material is continuously being added to the growing tip of the extending cell. Hyphae of fungi and oomycetes are analogous structures that look the same but have different wall properties. (Fungal walls are composed of chitin, whereas the walls of oomycetes are made of cellulose.)

We also examined four groups of fungi, the Chytridomycota (thought to represent the most ancestral state), the microsporidians, a group of intracellular parasites, the Zygomycota (a common bread mold), and the Glomeromycota, all of whose members are mycorhizzae in a mutualistic relationship with a land plant. Be sure you understand the life cycle of a zygomycete, and in the next tutorial you will learn how this life cycle distinguishes this phylum from others in the fungal kingdom.

Terms

After reading this tutorial, you should have a working knowledge of the following terms:

absorptive nutrition

arbuscular mycorrhizae (AM)

chytrid

coenocytic hyphae

dikaryon

fungi

gametangium (pl. gametangia)

glomeromycete

haustorium (pl. haustoria)

heterokaryon

hypha (pl. hyphae)

karyogamy

lichen

microsporidian

mycelium (pl. mycelia)

mycorrhiza (pl. mycorrhizae)

plasmogamy

septum (pl. septa)

septate hyphae

zygomycetes

zygosporangium (pl. zygosporangia)

zygospore

Case Study for Fungi I

Fungi of the genus Pilobolus live in the dung of herbivores (usually cows and horses) and are important recyclers of the nutrients in the dung. To complete their life cycle, the Pilobolus spores need to be ingested by an herbivore but grazing animals usually avoid grazing near dung. To be dispersed away from the dung and into a location where they are more likely to be eaten, Pilobolus produce sporangia that orient themselves toward the sun and then explode and send spores onto vegetation far away. When that vegetation is eaten, the spores are also eaten. The animal then excretes its dung and the Pilobolus spores which then germinate to produce the next generation. The presence of the fungus within the grazing animal is not known to have any negative effect on the animal.

Nematode lungworms of the genus Dictyocaulus cause parasitic bronchitis in grazing animals. They also need to be ingested by an animal in order complete their life cycle. Like Pilobolus they are passed through dung and their larvae need to move to clean vegetation to be ingested. To do this, they hitch a ride with the exploding Pilobolus. The presence of the lungworm larvae does not appear to have any negative effects on the Pilobolus but can cause death in grazing animals.

Pilobolus is classified as a member of the Zygomycota. What feature must it have that results in its classification as a zygomycete?

During reproduction, Pilobolus orients its sporangia toward the light. In plants and fungi, the word tropism describes growth in response to a particular environmental cue. What type of tropism is Pilobolus exhibiting?

What is the symbiotic relationship between Pilobolus and herbivores? Between Pilobolus and lungworms? Between the lungworms and herbivores?

Now that you have read this tutorial and worked through the case study, go to ANGEL and complete the tutorialpractice problemsto test your understanding. Questions? Either send your instructor a message through ANGEL or attend an online office hour (the times are posted on ANGEL).